Groundwater flow and residence times below a meso-tidal high-energy beach: A model-based analyses of salinity patterns and 3H-3He groundwater ages

Abstract

In this study, 3H-3He ages in the intertidal zone based on both field measurements and numerical simulations are presented to provide insights into the flow and transport behavior within the subterranean estuary of a meso-tidal, high-energy beach. In a systematic modelling approach, important hydrogeological parameters (dispersivity, hydraulic conductivity and fresh groundwater flux) were varied in a cross-shore transect at a beach on Spiekeroog Island in north-western Germany. Both measured and simulated salinity and age distribution within the intertidal zone suggest the existence of two– at least temporally occurring- saltwater recirculation cells accompanied by two fresh discharge location. This is in contrast to the state-of-the-art concept of a STE under tidal influence and is caused by a runnel and ridge system at the study site, which is a rather wide-spread type of beach morphology with topographical highs and interfering lows in the intertidal zone. Residence times within the two saline plumes were between weeks and several months. Freshwater from the island’s freshwater lens resided up to 18 years in the brackish beach aquifer before exfiltrating at the discharge points. Field measurements confirmed the discharge of decade-old water at the discharge points. Model results were highly dependent on the tested model parameters. A change between a one- and two-plume system, as well as overlapping conditions at the study site dependent on the change in beach topography are likely.